1. Field of the Invention
This invention relates to improvements in a compressed air powered paint shaking machine, and to a compressed gas powered mechanism for imparting a linear or rotational quasi-sinusoidal vibration or oscillatory motion to various loads.
2. Description Of The Prior Art
Prior pneumatically driven paint shaking machines have typically been powered by single-acting air cylinder motors, and have relied on coil springs for reversing the motion imparted by the motor piston. See, for example, U.S. Pat. No. 3,301,534, issued Jan. 31, 1976, to L. D. Orser for "Paint Shaker Machine". Valving for this type of machine has been either a simple piston-actuated poppet air admission valve with exhaust ports in the cylinder wall, or a link-operated variation of the ancient steam chest slide valve. In either case, the machine was limited as to power, speed, efficiency and reliability. Because the return springs were linear, a compromise had to be accepted between the need for a stiff spring to provide high speed and powerful shaking action, and the need for a soft spring to promote ease of starting. Further, a stiff spring in turn required a more powerful air cylinder motor, making starting very difficult in the case of a poppet valve type, and making for unacceptably high air consumption or difficult starting with the slide type valve, owing to inherent valve timing limitations. When operated at recommended supply pressures, such shakers will not adequately and quickly shake a large can of paint, and when operated under overpressure conditions, these shakers are unable to control their motions without collisions of internal parts which leads to damage and a shortened machine life.
Modern metallic paints contain powdered aluminum which settles to the bottom of the paint container during storage and forms a heavy cake or sludge on the bottom of the container. It is difficult to re-suspend completely the metallic particles without a highly energetic shaking motion. Prior pneumatic shakers have not performed adequately as metallic paint mixers. The powerful electric motor-powered shakers which have performed adequately have been heavy, bulky, and expensive.
An associated problem with prior machines was that the valving arrangements used were not suitable for higher speed operations. Poppet valve controlled machines seemed to promise high efficiency at high speed because their compression stroke provided a pneumatic spring effect and incidentally precompressed the air in the cylinder to a pressure just below that of the compressed air supply under design conditions. This resulted in low flow losses upon gas admittance and expansion was nearly adiabatic. All this promised good efficiency, which was obtained, but only in a narrow operating range. Starting the machine required manually precompressing the cylinder until the piston contacted the poppet valve to admit compressed air, and higher compression ratio or larger diameter cylinders made starting difficult and potentially more dangerous. It would have been necessary to increase compression ratio or cylinder size in order to increase the speed and power of the machine. The machines with linkage operated valves had somewhat more design flexibility but the valve timing and design changes which would promote easier starting of larger cylinders would also reduce efficiency and maximum speed. Since compressed air system efficiency is, at best, not very high owing to thermodynamic losses in compression as well as expansion, and energy costs are becoming significant, large efficiency reductions can have a serious impact on the market value of the air-powered machine. Also, rough mathematical models of mixing ability in paint shakers show that mixing power is, in the microscale, dependent on the frequency of oscillation times the maximum liquid shear rate. This means that microscale mixing is roughly proportional to the third power of the speed of a given fixed-amplitude machine, making speed very important to mixing performance. A high speed, high efficiency, easy starting valving arrangement was needed.